Unusual cases of Anti-SRP necrotizing myopathy with predominant distal leg weakness and atrophy.

Anti-SRP necrotizing myopathy is classically characterized by subacute or chronic, severe, progressive and symmetric myositis which predominantly affects proximal muscles. We report two unusual cases presenting with predominantly distal, asymmetric weakness, with selective involvement of the posterior compartment of the thighs, gastrocnemius, and soleus muscles, in addition to inflammation and edema on STIR or T2-weighted, fat-saturated MRI. In each case, creatine kinase (CK) levels were >10 times normal and myositis panels returned positive for anti-SRP. ANA, ENA, RF, and HMGCR antibody were all negative. Nerve conduction study (NCS) was normal. Electromyography (EMG) confirmed diffuse myopathy with fibrillation potentials and positive sharp waves. Additional work up, including whole exome sequencing (WES), immunohistochemical staining for different types of muscular dystrophy, and western blot for calpain 3 and dysferlin were negative. The strength and CK levels of both patients markedly improved following immunosuppression. Our cases emphasize the importance of considering anti-SRP necrotizing myopathy in patients presenting with recent onset predominant asymmetric distal leg weakness of unclear etiology, and support the usefulness of MRI of the distal legs for early recognition. Given the potential consequences of delays in treatment of this condition, the recognition of this clinical pattern is important and can allow for prompt initiation of aggressive immunotherapies.